DIY Header flange thickness?

Looking at making header flanges for my 300 LSx head.  Easy button would be to get a couple of eBay flanges and cut them up, may be a decent idea as well as that would allow for less strain on the flanges as the head expands and contracts.  However the easy button would end up as a hit on the Challenge budget when I could just get some bar stock.

So I was going to pick up some bar stock, but I don't know how thick to go.  1/4" seems like it is too small.  3/8" is all over the place on eBay, so that may be the way to go.  My usual inclination to overbuild is rearing its head, though and I'm wondering about 1/2"

Other things to consider with this?

Best luck I've ever had with headers has been a set from Banks on my Ford 460 and a set from Gibson on a Vortec 454. Both have 5/16" flanges and both stayed tight and leak free the whole time I've owned the vehicles.

Both were used for heavy towing. We still have the 454 

For my Seven, I cut the flange off a stock exhaust manifold - 1.6 Miatas use tubular manifolds, so that was easy. Still nice and tight after a bunch of track work. They're not very thick and the price sure was right.

For the Targa Miata header...same thing. (had to look it up). I flattened it on a belt sander after construction. Stayed night and tight after a whole bunch of track use.

And I'll just leave this here as a reminder to why I can't be left alone.

Mine are 5/16", but mainly because that's what my company happened to be cutting that day... I agree 1/4" is probably too thin unless you plan on welding the runners with the flanges bolted and clamped flat to something very rigid. My 5/16" flanges tried to peel up in the corners from the runner-to-flange welds, even though I was clamped to a large piece of angle iron.

For your I6, I would recommend NOT splitting the flange like I did here. I had issues with the two flanges warping away and at funny angles from each other. A single-piece flange would have saved me a ton of effort. If I had to do it again, I would have made a single flange from 3/8".